1. Field of the Invention
This invention relates to the surface treatment of component parts extensively used in various industries manufacturing heavy and light electric machinery and apparatus, household electric appliances, and light and heavy machinery, and in building and construction industries, as well as in automobile, railroad, aircraft, and other transportation industries and, more specifically, to the surface treatment of component members, especially members based on metals, that are required to possess corrosion resistance and fine outward appearance in addition to the properties to be imparted by the surface treatment.
2. Prior Art
Surface treatment with zinc has been a classic method of protecting ferrous materials and component parts against corrosion. To meet a growing demand for enhanced corrosion resistance, surface treatments for improving the zinc treatment have been studied and developed since about a decade or two ago. The new approaches include treatments using zinc alloys, such as zinc-iron, zinc-nickel, zinc-cobalt, zinc-manganese, and tin-zinc alloys, and composite treatments using zinc-silica and the like. Those techniques are more or less effective but still have difficulties to be overcome to comply with more severe requirements in recent years for improved performance. In the art of alloying it is known that increases in the codeposition percentages of iron and nickel as alloying elements (about 13–14% nickel being known to be the best) improve the corrosion resistance of the alloy. In reality, however, alloy plating is done, for example, with a zinc-iron alloy having an iron codeposition percentage of not more than 1% and with a zinc-nickel alloy having a codeposition percentage of about 5–7%. The reasons are that, if the codeposition iron percentage is increased (beyond 1%) in order to meet the demand for greater corrosion resistance, the plating can blister, come off, or otherwise fail to adhere securely upon subjection to load (by secondary operation), such as bending, spiraling, extrusion, indentation, impacting, or rolling of the surface treated work, or upon heating or with the lapse of time. The surface treated members that have had such troubles are no longer of any value as such in respect of corrosion resistance or ornamental effect.
Today, treatment with zinc-nickel alloys having a nickel codeposition percentage of about 13% is in practice in part of the world. The treatment is still unable to provide a basic solution of the afore-described problem of inadequate adhesion on secondary operation. It is hardly applicable to objects whose plating adhesion is challenged by bending, spiraling, extrusion, indentation, impacting, rolling, or other secondary operation after the surface treatment. Another problem is the buildup of waste bath constituents during running, which leads to a drop of current efficiency and hence lower productivity. Among many other problems are the severity of controlling the treating conditions to maintain a narrow codeposition percentage range and the difficulties involved in disposing of the wastewater due to the presence of waste-containing organic matter.
Composite plating with a zinc-silica alloy or the like is being researched in some sectors of industry, primarily for the treatment of sheet steel, but the cases of actual application to component members are only a few. This is because the technique for sheet steel differs widely from that for members; a technique for treating a flat sheet steel is unable to plate evenly members of complex configurations. Moreover, a sheet steel treatment with a zinc-silica system is not directly applicable to component members since it provides an outward appearance inferior in fineness and luster, due to substantial irregularities of the treated surface for which silica is responsible and also to uneven distribution of silica particles about 0.1 μm in size that coagulate in the matrix. The zinc-silica composite treatment imparts improved corrosion resistance with an increase in the silica content. On the other hand, an increase in the deposit further deteriorates the outward appearance of the treated surface, and this makes the composite treatment less suitable for the surface treatment of component members. Thus the composite treatment is practically unable to establish compatibility between high corrosion resistance and fine outward appearance.
By way of example, Japanese Patent Application Kokai No. 61–143597 describes in its Example 2 a zincate plating solution to which fine silica particles are added. The plating solution cannot be used in the plants for the surface treatment of component members to which the present invention is applicable. The reasons include: (1) suspension of minute silica particles in the plating solution, and (2) the minute silica particles present in the plating surface produce surface unevenness and thereby mar the appearance. In the plants where component members are plated, the plating solution tanks are provided with many lines for connection with filters and circulating pumps to maintain the purity and temperature of the solution. If minute silica particles are suspended in the plating solution, they readily clog the filters and get them out of use, rendering it difficult to keep the solution clean. Choked lines would not only make it impossible to maintain the solution at a predetermined temperature but also destroy pumps and other facilities in extreme cases. It might be considered possible to reduce the proportion of minute silica particles so as to enhance the appearance of the treated surface. However, as will be understood from a comparison of the above Example with the rest of examples of the invention (Pat. App. Kokai No. 61-143597), a decrease in the silica content in a plating is accompanied with a corresponding decrease in the corrosion resistance, and it is an attempt at merely reducing the advantageous effect of the cited invention. By way of reference it may be added that, whereas the plating according to Example 2 is 18 μm thick, a thickness range of 5–8 μm is predominant for ordinary platings for component members. In this sense the thickness range of the cited invention differs to a substantial extent from the practical range. The corrosion resistance life of a zinc plating until red rusting is generally proportional to the thickness of the plating. If the plating performance value in Example 2 of the invention (Pat. App. Kokai No. 61-143597) is simply multiplied by 5/18 and the plating thickness is converted to 5 μm, then the period of time until red rusting will be 66.7 hours. Since ordinary zinc plating is said to be corrosion-resistant for 7.5–8 hours per μm of thickness, it follows that a plating 5 μm thick has a pre-rusting duration of about 40 hours. From this slightness of performance difference it is manifest that a further decrease of the minute silica particle content will have the danger of eliminating the effectiveness of the Pat. App. Kokai No. 61-143597 upon ordinary zinc plating. In other words, as the cited invention stands, the minute silica particle content cannot be increased because it will further deteriorate the outward appearance nor can be decreased for fear of reduced corrosion resistance.
In view of these problems, another invention was made to avoid the suspension in a plating solution (i.e., to make the solution clear through dissolution of silica) by restricting the concentration of caustic soda and the proportion of silica particle seeds. At laboratory level the invention in question settled the problems of plating appearance and suspension in plating solution. At actual site in a plant the suspension in plating solution sometimes occurred during extended non-operation time such as year-end and New Year's holidays. Thus the second invention too presented the problems of the Pat. App. Kokai No. 61–143597 or could not preclude that possibility. It is easily expected that the second invention cannot solve the problem of suspension during downtime by the addition of nickel, cobalt, or other metal, and in fact the invention has proved that it cannot. In addition, component members plated with a solution of the invention, with the addition of iron, tended to have the uneven appearance that is given by a silver-free surface treating agent; the members could not have a finely black colored surface unless silver was used.
It is an object of the present invention to settle the problems of the prior art that have been left unsolved. Zinc alloy plating and composite zinc plating have hitherto been studied in order to comply with the growing demand for higher corrosion resistance. In the case of zinc alloy plating, an increase in the alloy component concentration will increase the alloy proportion in the plating and improve the corrosion resistance. However, the plating will not serve its purpose because of deterioration in adhesion and other physical properties (while, of course, reduction of the alloy proportion will lower the corrosion resistance). In composite zinc plating, an increase in the dispersant concentration (an increase in the precipitate proportion in the plating) will enhance the corrosion resistance but will further affect the outward appearance that is originally inferior (while a decrease in the dispersant concentration naturally deteriorate the corrosion resistance).
It is another object of the present invention to overcome this dilemma and provide a plating solution, plating method, and surface treating agent that will impart higher corrosion resistance, better physical properties, and finer appearance than heretofore.
A further object of the invention is to provide a surface treatment which is easier to control than before in preventing the suspension or settlement of the plating solution, in maintaining a broader temperature range, and in avoiding the deterioration performance in the course of running, while giving a fine black appearance more easily than conventional techniques.